@ displayed incorrectly in javadocs

This is how it's displayed when rendered: http://junit.org/javadoc/latest/org/junit/Assume.html

Used the same escaping strategy as in RunWith's javadocs source (https://github.com/junit-team/junit/blob/master/src/main/java/org/junit/runner/RunWith.java)
Merge pull request #849 from kryger/patch-1

@ displayed incorrectly in javadocs